If the truck has had synthetic oil in it for a long time, I'd recommend continuing to use it. The problem with switching between petroleum based oil and synthetics is that the engine seals become saturated with whatever you are using and may develop leaks if you switch. This is more common when switching TO a synthetic oil, but the same principle applies both ways. If you are not running an oil filter (as in the case of many old stovebolts) you probably won't realize any benefit from synthetics since you must change the oil much more frequently. I've been running Shell Rotella T in my 235 which was a recommendation I got here. It has worked out great and is comparable in price to other petroleum based oils.
